Our goal is to compare B-lymphocyte function in persons of different ages. As T-lymphocyte function is impaired in aged humans, care will be taken to distinguish between an impaired PFC response that arises as a consequence of the age-associated T-cell defect and an age-associated defect inherent to the B-cell population. In addition, the contribution of altered monocyte function to the impaired immune response will be studied. The procedure we will use to assess B-lymphocyte is the generation of PFC in vitro by human B-lymphocyte preparations. PFC will be induced by cultured B-lymphocytes with: (a) the Cowan strain of staphylococci, a polyclonal B-cell activator; (b) a T-cell mitogenic factor prepared from allogenic mixed lymphocyte culture; (c) a macrophage factor obtained from monocyte cultures incubated with lipopolysacconide (LPS). The capacity of soluble T-cell factors from young subjects to complement the function of T-cell depleted human mononuclear cell preparations from old and young subjects will be tested to establish whether a defect is intrinsic to the B-cell population. In this way, the cellular basis of the impaired immune reactivity of aged humans will be determined.